Low-frequency high-energy ultrasound refers to ultrasound having an operating frequency of 15 to 100 kHz, preferably 15 to 60 kHz, e.g., 20 kHz, and a sound output above 10 Watts, preferably 100 Watts to 20,000 Watts, e.g., 8,000 Watts. For example, piezoelectric or magnetorestrictive systems are used to generate ultrasound. Linear sound transducers and planar or arcuate plate-type vibrators or tube resonators have been known. Low-frequency high-energy ultrasound is mostly used in the treatment of liquids such as, for example, foodstuffs, cosmetics, paints and nano-materials. The effect of low-frequency high-energy ultrasound on treated liquids is predominantly based on cavitation occurring in the liquid. During the sonication of liquids, cavitation occurs. As is known, this results in the wear or erosion of the surface(s) of the sonotrode(s) which transmit vibrations of a higher amplitude (oscillation-transducing sonotrode surface) into the liquid medium.
The eroded material of the sonotrode material, e.g., titanium, is released into the sonicated liquid. Thus, the erosion results in replacement costs for the sonotrodes and in the contamination of the liquid that is to be sonicated.
From the past, various methods for reducing or suppressing material erosion have been known.
Referring to document DE 199 38 254.9, a pressure that completely or largely suppresses cavitation in a reactor vessel is generated in a reactor vessel by sonication of said reactor vessel with low-frequency high-energy ultrasound. As a result of this, wear and material erosion are significantly or completely reduced; however, also the cavitation effect on the liquid is considerably or completely reduced.
Referring to document DE 2004 025 836 A1, a method has been known with which the erosion due to cavitation on the oscillation-transducing sonotrode surface itself is to be reduced considerably or completely by means of a layer between the sonotrode and the sonicated medium. However, erosion occurs on the oscillation-transducing layer, thus necessitating a contouring/replacement of this layer.
Also known have been solutions for focusing the low-frequency high-energy ultrasound on a region that is far remote from the oscillation-transducing sonotrode surface. The disadvantage is the large spatial expansion of the device used for focusing.
Inasmuch as, for example, the wavelength of sound in water at 20 kHz is, for example, 60-80 mm, and the reasonable size of a lens should be at least 10 times the wavelength, an ultrasound lens for focusing would have a diameter of more than 600 mm. In addition, cavitation would interfere with the sound field.
It is possible to inject low-frequency high-energy ultrasound in a coupling liquid such as, for example, water; see, e.g., DE 102 43 837.4. This coupling liquid transmits the vibrations to a vessel wall, e.g., glass, whereby the liquid to be sonicated is located on the opposite side of said vessel wall. This leads to a loss of efficiency and thus to a reduced intensity of sonication. Wear/erosion occurs on the vibrating vessel wall.
Coating or hardening the surface of the sound-transmitting surface achieves a delay or a reduction of wear. However, any coating is expensive and its effect is limited by time. In addition, anything that is worn off the applied material ends up in the liquid.